The present invention relates generally to electrically heated liquid heaters. More particularly, the present invention relates to methods and device for adjusting electrical power to an electric resistance heating element in the liquid heater in a manner to either reduce its average electrical power or to allow the liquid heater to receive a significant quantity of heat from an auxiliary heat source.
A hot water heater typically includes a vertically mounted cylindrical water tank, a cylindrical shell coaxial with and radially spaced apart from the water tank to form an annular space between the outer wall of the water tank and the inner wall of the shell, and insulating at least a portion of the annular space for providing thermal insulation to the water tank. Polymer foam expanded directly within the annular space is an effective insulating material.
The typical water tank has various appurtenances such as inlet, outlet and drain fittings. Especially, the water tank is provided with water heating and temperature control means. Typically for electrically heated water heaters, the water heating means comprises one or more electrical resistance heating elements. Each heating element extends through a fitting in the wall of the water tank such that the resistance heating element is inside the tank and means for connecting the resistance heating element to an electrical power source is outside the tank.
Electric water heaters with storage tanks between 30 and 120 gallons typically have two electric-resistance elements that heat the stored water. One element is located near the bottom of the storage tank and the second is located at a height approximately one-fourth to one-third down from the top of tank. Both elements commonly have the same heating rate. Although elements for 240 V (the voltage at which most 30 to 120 gallon water heaters operate) electric water heaters are available with heating rates between 750 W and 6000 W, 4500 W elements are most commonly installed at the factory. This gives the water heaters a high reheat capability without requiring wiring changes to the building to handle more than 20 A on the water heater""s circuit.
The temperature control means for an electrically heated water heater commonly comprises a mechanical thermostat which operates a switch to apply electrical power to the electrical resistance heating element when water in the tank is sensed to be below a selected set point temperature, and operates the switch to disconnect electrical power from the electrical resistance heating element when the water in the tank is at or above the set point temperature. With such temperature control means, electrical power to the electrical resistance heating element is either full on, passing full electrical current, or completely off.
Electric water heaters with storage tanks almost always operate with stratified thermal conditions inside the tank. Hot water is drawn from the top of the tank, while at the same time, cold water enters near the bottom. Since the cold water is more dense than the hot, it tends not to mix with the hotter water above.
Typically, when a hot-water draw occurs, the tank""s lower thermostat will be the first to sense the cold water entering the tank. This triggers the lower heating element. If a large volume of hot water is quickly drawn from the tank, the level of cold water within the tank can reach the upper thermostat. This will simultaneously trigger the upper element and turn off the lower element. The upper element reheats the top 25% to 33% of the tank. Once the upper thermostat has been satisfied (i.e., the top of the tank has been reheated), the upper element turns off and the lower element resumes heating the remainder of the tank.
Electric resistance water heaters are generally simple and inexpensive devices. However, such heaters are expensive to operate and have a very high instantaneous demand for power in comparison to their average power demand.
The US Department of Energy""s rating procedure for water heaters assumes a daily average consumption of hot water in residences of 64.3 gallons per day. Assuming that the hot water is heated from 65xc2x0 F. to 130xc2x0 F., this usage corresponds to an average daily power of 423 W. However, since a water heater""s upper and lower elements are both typically 4500 W, its instantaneous electrical demand will typically be ten times its demand averaged over 24 hours. Furthermore, there is a high level of coincidence in the operation of residential water heaters within the same geographical region. For most homes, hot-water use is highest in the morning when people wake up and take showers. It is common for an electric utility to have an average (or diversified) demand from all the water heaters within its service territory of 1,500 W during weekday mornings. If these water heaters could be controlled so that most never operate at a power much higher than the average required to meet the daily use of hot water, the morning weekday peak could be reduced by about 1,000 W per water heater.
One approach to reducing the operating cost for a water heater is to supplement its operation with an auxiliary heat source such as a desuperheater, a solar thermal collector, a heat pump, and the like. Typically, such a heat source is attached to the water heater so that it draws water from a location near the bottom of the tank, heats the water, and then returns the water to a location near the bottom of the tank.
One of the problems associated with auxiliary heat sources is that their heating rate is typically much lower than the heating rate associated with the resistance elements that come with the water heater. Thus, the heating contribution of such heat sources will be greatly diminished if the resistance elements in the water heater are allowed to simultaneously operate.
The present invention is generally directed to a device for controlling the power consumption of one or more electric resistance heating elements in a vessel containing a heatable liquid (such as water), at least one inlet for receiving a liquid having a first temperature, at least one outlet for removing liquid having a second higher temperature, and at least one thermostat for directing power to at least one heating element. The device of the present invention comprises detection means for detecting one or more predetermined conditions in said vessel relating to the amount of liquid having said second higher temperature remaining in said vessel, and for generating an initiating signal corresponding to said predetermined condition, controller means responsive to the initiating signal for outputting a corresponding switching signal and power modulating means responsive to said switching signal for modulating power to at least one heating element.